1. Field of the Invention
This invention relates to ink jet printing and particularly to a method and apparatus for controlling the velocity of ink drops in an ink jet printer.
2. Description of the Prior Art
In ink jet printers of one well-known type, drops of a field-controllable ink are formed and propelled from a nozzle toward a print medium. Ink is supplied to the nozzle under pressure sufficient to cause the ink to issue from the nozzle as a continuous stream. A drop generator such as a piezoelectric or magnetostrictive vibrator attached to the nozzle or other means located adjacent the continuous portion of the stream generates perturbations in the stream to cause it to break into individual drops of substantially uniform size and spacing. Field control devices located in the vicinity of the stream are regulated in accordance with the data signals to cause individual drops to be dispersed onto the print medium in the form of the desired data pattern. To insure proper placement of the drops it is important that the velocity of the drops be maintained as constant as possible.
The need for maintaining the velocity of the ink drops substantially constant to insure good print quality is well-recognized in the art. A discussion of prior art is contained in the above-mentioned cross-reference related application. In the related application an invention is described and claimed which is directed to a method for making a coarse correction in the velocity of the ink drops when a gross velocity error exists. The invention of the related application basically utilizes a change in the drop generation frequency to detect presence of a gross velocity error and then making a coarse correction followed by generating drops at the printing frequency and making a fine velocity correction in the event a further velocity error exists. In certain applications of ink jet printers it may not be practical to alter the drop generation frequency to determine if a gross velocity error exists. The present invention is directed to a method for making velocity corrections where a gross velocity error can exist but which does not require a change in the drop generation frequency.